Street Fighter IV Flashback was a concept created by Backbone Entertainment in 2006, to try to pitch a full project at Capcom. It’s not that strange that Backbone proposed something like this, as they are in good relationship with Capcom: they developed various games published by Capcom, like 1942: Joint Strike, Wolf of the Battlefield and the Super Street Fighter II Turbo HD Remix. Also, keep in mind that this concept is not related to the real Street Fighter 4, which development started only much later.
Sf4: Flashback was meant to be a 3D sequel to the Street Fighter series, with the classic 1 vs 1 fights, online play and a full single player adventure mode, know as “Ryu’s Journey”. Ryu’s Journey was supposed to be a free-roaming 3D action game, in the vein of Ninja Gaiden or Onimusha, with levels to explore, lots of enemies to fight, NPC to talk to and final bosses.
The concept behind the “Flashback” had various meanings. The single player mode was set in Ryu’s flashback to SF2 and the story would have followed classic Street Fighter moments and revisit familiar characters. Also, there was meant to be a new game mechanic called “flashback ” that let the players to rewind time to undo mistakes.
The concept arts of Street Fighter IV Flashback were made in collaboration between Backbone, Massive Black and UDON (that in the end worked on the SSFII Turbo HD Remix arts). Unreal 3 would have been used as the main 3D engine for the single and multiplayer modes.
Even if the project could have been interesting, this concept was not approved by Capcom: probably not even a playable prototype was ever made and only these few concept artworks / 3D models remain to preserve the existence of this game that will never be.